This invention relates to the field of electrostatic imaging and, more particularly, to the preparation of liquid toners containing components for improving the stabilization of the electrical properties due to the charge directors contained therein.
In the art of electrostatic photocopying or photoprinting, a latent electrostatic image is generally produced by first providing a photoconductive imaging surface with a uniform electrostatic charge, e.g. by exposing the imaging surface to a charge corona. The uniform electrostatic charge is then selectively discharged by exposing it to a modulated beam of light corresponding, e.g., to an optical image of an original to be copied or to a computer generated image, thereby forming an electrostatic charge pattern on the photoconductive imaging surface, i.e. a latent electrostatic image having a background portion at one potential and a "print" portion at another potential. The latent electrostatic image can then be developed by applying to it charged pigmented toner particles, which adhere to the "print" portions of the photoconductive surface to form a toner image which is subsequently transferred by various techniques to a copy sheet (e.g. paper).
It will be understood that other methods may be employed to form an electrostatic image, such as, for example, providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. The charge may be formed from an array of styluses. This invention will be described in respect of office copiers and the like, though it is to be understood that it is applicable to other uses involving electrostatographics including electrostatographic printing.
In liquid-developed electrostatic imaging, the toner particles are generally dispersed in an insulating non-polar liquid carrier, generally an aliphatic hydrocarbon fraction, which generally has a high-volume resistivity above 10.sup.9 ohm cm, a dielectric constant below 3.0 and a low vapor pressure (less than 10 torr. at 25.degree. C.). The liquid developer system further comprises so-called charge directors, i.e. compounds capable of imparting to the toner particles an electrical charge of the desired polarity and uniform magnitude so that the particles may be electrophoretically deposited on the photoconductive surface to form a toner image.
In the course of the process, liquid developer is applied to the photoconductive imaging surface. Under the influence of the electrical potential present in the latent image and a developing electrode which is usually present, the charged toner particles in the liquid developer film migrate to the "print" portions of the latent electrostatic image, thereby forming the developed toner image.
Charge director molecules play an important role in the above-described developing process in view of their function of controlling the polarity and magnitude of the charge on the toner particles. The choice of a particular charge director for use in a specific liquid developer system, will depend on a comparatively large number of physical characteristics of the charge director compound, inter alia its solubility in the carrier liquid, its changeability, its high electric field tolerance, its release properties, its time stability, the particle mobility, etc., as well as on characteristics of the developer. All these characteristics are crucial to achieve high quality imaging, particularly when a large number of impressions are to be produced.
A wide range of charge director compounds for use in liquid-developed electrostatic imaging are known from the prior art. Examples of charge director compounds are ionic compounds, particularly metal salts of fatty acids, metal salts of sulfo-succinates, metal salts of oxyphosphates, metal salts of alkyl-benzenesulfonic acid, metal salts of aromatic carboxylic acids or sulfonic acids, as well as zwitterionic and non-ionic compounds, such as polyoxyethylated alkylamines, lecithin, polyvinyl-pyrrolidone, organic acid esters of polyvalent alcohols, etc.
Notwithstanding the undoubted utility of charge directors, however, the charging caused thereby is generally unstable. In particular, lecithin, basic barium petronate (BBP) and calcium petronate (CP), which are used as negative charge directors, are unstable under high voltage conditions. Thus, when a solution of charge director (or a dispersion of toner particles in carrier liquid and containing charge director) is subjected to a high electric field, e.g. during the development process, the charge transport characteristics and conductivity suffer from transient suppression, and it may take several minutes for these characteristics to recover. This leads to unstable printing performance when long print runs are undertaken. Further, such solutions or dispersions containing particularly BBP, CP and to a lesser extent lecithin, tend to lose conductivity in the course of time (after dilution with Isopar or other carrier liquids), so that, e.g., solutions or dispersions containing BBP or CP, when diluted with Isopar, will change their conductivity by about one order of magnitude in a day and a half. In this connection, it may be noted that in U.S. Pat. No. 4,897,332 (Gibson), there is described the use of alkylated polyvinylpyrrolidones in liquid toners, for the purpose of promoting their electrical stability under high voltage conditions.
In an attempt to improve the quality of the image formed, particularly when using liquid toners containing charge directors, it has been suggested to use adjuvants in the toner compositions, such as polyhydroxy compounds, aminoalcohols, polybutylene succinimide, an aromatic hydrocarbon, a metallic soap or a salt of a Group Ia, Ia, or IIIa metal.
In U.S. Pat. No. 3,681,243 (Okuno), the problem of stained prints in electrophotography with liquid toners, said to be due to lack of smoothness of the relevant surfaces and lack of uniformity of electrical charge, is stated to be solved by use of an additive, which may be a C.sub.12-16 saturated monocarboxylic acid. According to Okuno's disclosure, the resin in the toner is a "polar-controlling resin", e.g. "Nikanol HP-100", the principal component of which is said to be phenol modified xylene resin. However, in this U.S. Patent, there is no explicit reference to the presence of a charge director.
U.S. Pat. No. 4,891,286 (Gibson) claims that pigment charge homogeneity and mobility in liquid toners are improved by the addition of carrier-liquid insoluble monomeric organic acids to the toner solutions. This patent stresses that the acids must be associated with the pigment-binder particle and must not exist free in the continuous liquid phase.
U.S. Pat. No. 5,002,848 (El-Sayed et al) discloses positive-working liquid developers, said to have improved charging characteristics, which consist essentially of (A) a non-polar liquid, (B) thermoplastic resin particles charged positive, (C) a nonpolar liquid soluble ionic or zwitterionic charge director compound, and (D) a substituted carboxylic adjuvant, which may be, in particular, (i) an alkane- or aryl-carboxylic acid, substituted by an electron-withdrawing group, or by a carboxylate anion-stabilizing moiety (e.g. OH, SH or thioether) in the a-position of an alkane carboxylic acid, (ii) an arylcarboxylic acid ortho-substituted by a carboxylate anion-stabilizing moiety, (iii) an analogously substituted alkylarylcarboxylic acid. This patent does not teach that ingredient (D) stabilizes the electrical properties of ingredient (C), or that there is any merit in the particular order, in which the various ingredients of the liquid toners are mixed together, in relation to stabilizing the electrical properties of the charge directors present.